project name:	leslie
project url:	https://github.com/moapacha/leslie
author:	sym
description:	doppler, tremolo, and the slow spin-up
documentation url:	https://github.com/moapacha/leslie/blob/main/README.md
tags:	audio_fx engine

leslie

leslie running on norns

A norns rotating-speaker effect modeled on the dual-rotor Leslie cabinet. Stereo input is split at a Linkwitz–Riley crossover into a horn band (high) and a bass-drum band (low). Each band is run through a doppler delay, an AM tremolo, and a stereo pan, all driven by counter-rotating LFOs. Slow / fast / brake modes follow the classic Leslie 122 — speed transitions are smoothed so rotors visibly and audibly spin up or coast to a stop.

Requirements

norns or norns shield
a stereo (or mono) source patched into the audio inputs

Controls

Input	Function
E1	input drive (0.25–4.00, soft saturate above 1.0)
E2	throb — AM depth (0.00–1.00, both rotors linked, bass at 0.8×)
E3	stereo width (0.00–1.00)
K2	toggle slow / fast (chorale ↔ tremolo)
K3	brake — smooth spin-down to 0; press again to release
K1 (hold)	edit mode for fine-tuning the deeper params

While K1 is held the right column lights up and the encoders re-route:

Input	Function
K1 + E1	doppler depth, both rotors (0–5 ms; bass scales 2× automatically)
K1 + E2	rotation rate of the currently selected mode (0.01 Hz steps)
K1 + E3	crossover frequency (200–2000 Hz, 1 Hz steps)

Every value on screen is also exposed in the PARAMS menu under leslie, where horn and bass parameters can be set independently and the accel times tweaked.

OLED layout

Region	Content
Left (x 4–40)	cabinet — vent slats, horn rotor (bowtie of two flared bells), bass drum, legs
Title row	`~leslie~`, with a `*` badge at the top-right corner while K1 is held
Mode row	`slow` / `fast` / `brake` — current mode at level 15, others dim
Param rows	left column = normal-mode params (`drive`, `throb`, `width`); right column = edit-mode params (`dop`, `spd`, `xov`). The active set is bright, the other dim, so all six values are visible at once.
Tach	`h` and `b` show the target rate of the current mode so the display reflects mode changes instantly. The on-screen rotors lerp toward that target with a per-rotor accel time, mirroring the engine's `Lag.kr`.

The horn flare width modulates with abs(cos(angle)) so the bowtie thins to a line when end-on and opens back up when broadside, suggesting a vertical-axis rotation.

Signal chain

A single \leslie SynthDef in lib/Engine_Leslie.sc:

Stage	What it does
input	sum stereo to mono, multiply by `drive`, soft `tanh` saturate
crossover	LR4 (cascaded LPF / HPF) at `xfreq` — sums flat, no magnitude dip at the split
rate smoothing	`Lag.kr(hornRate, hornAccel)` and the bass equivalent. `brake` is multiplied in after the lag, so engaging brake decelerates smoothly from whatever the current rate is and releasing it accelerates back.
LFOs	`SinOsc.kr(hornCur)` for the horn, `SinOsc.kr(-bassCur)` for the bass — counter-rotating
doppler	each band fed through `DelayC.ar` with delay time `(lfo × dopp) + offset`. The varying delay produces the swirling pitch shift.
AM	each band amplitude-modulated by its own LFO at `1 − depth/2 ± depth/2`, so depth=1 swings between near-zero and full amplitude
pan	each band panned by its own LFO scaled by `width`. Because the bass LFO counter-rotates, the two bands sweep in opposite directions at independent rates.
mix	wet / dry crossfade

Defaults track the Leslie 122: chorale 0.8 Hz horn / 0.7 Hz bass, tremolo 6.8 Hz horn / 5.6 Hz bass, crossover 800 Hz, horn accel 0.5 s, bass accel 1.2 s.

Install

From maiden:

;install https://github.com/moapacha/leslie

Or clone into ~/dust/code/leslie/ directly. The first load will compile the SuperCollider engine; if the engine does not appear, run ;restart in maiden to rebuild the class library.

Credits

The signal chain follows the architecture of mda Leslie (Paul Kellett, GPL), translated to SuperCollider for norns Crone. Defaults reference the Leslie 122 cabinet.

Hope you enjoy this script. Suggestions and contributions are welcome.